Variable resistance element and semiconductor storage device

ABSTRACT

A variable resistance element is formed by sandwiching a metal oxide layer whose resistance changes between a pair of electrodes and the metal oxide layer includes a pair of variable resistance layers whose resistances change by formation of a current path and a branching suppression layer which is sandwiched between the variable resistance layers and suppresses branching of the current path.

TECHNICAL FIELD

The present invention relates to a variable resistance element and asemiconductor storage device and in particular relates to a resistancechange type non-volatile variable resistance element and a semiconductorstorage device.

BACKGROUND ART

The most common non-volatile memory in the market today that isrepresented by a flash memory, a SONOS(Silicon-Oxide-Nitride-Oxide-Silicon) memory, or the like uses atechnology by which a threshold voltage of a semiconductor transistor ischanged by storing an electric charge in an insulating film formed on achannel.

In order to realize a large capacity non-volatile memory, a fine elementstructure has to be developed and used. However, a today's fineprocessing technology almost reaches a limit of processing or the like.Accordingly, it is difficult to miniaturize even a single semiconductortransistor.

Accordingly, an idea in which the transistor has only a function toselect a memory cell used for reading and writing (acts as a switchingelement) and a structure in which a storage element and the switchingelement are separated from each other like a DRAM (Dynamic Random AccessMemory) is used is considered to realize a large capacity non-volatilememory.

In this case, a variable resistance element whose electric resistancevalue changes between two or more values when some electrical stimulusis given may be used.

However, an electrical characteristic required when the variableresistance element is used as the storage element is different from thatrequired when the variable resistance element is used as the switchingelement for performing a switching between the wirings.

Namely, when it is used as the storage element, because the variableresistance element is connected in series to an active element such as atransistor or a diode which selects the storage element, a resistancevalue of the variable resistance element may be about 1 kΩ in alow-resistance state and it is required to be about 100 kΩ in ahigh-resistance state. Namely, the resistance value of the variableresistance element has to be changed by two orders between an ON stateand an OFF state.

In contrast, when the variable resistance element is arranged betweenwirings L1 and L2 to configure a switch Sw as shown in FIG. 9, it isrequired that the resistance value of the variable resistance element inthe low-resistance state is equal to a resistance value (for example,100Ω or less) of the wiring and the resistance value of the variableresistance element in the high-resistance state is 100 MΩ or more tosurely cut off a signal.

Accordingly, the variable resistance element has to be produced so thatit has an electrical characteristic according to the use.

The various structures are proposed for the variable resistance element.FIG. 10 is a schematic sectional view of a variable resistance element50 having a metal/metal oxide/metal (hereinafter, referred to as MIMtype) structure in which a metal oxide is sandwiched between electrodes.In this variable resistance element 50, a metal oxide film 51 with aresistance change characteristic is arranged between electrodes 52 and53 that are made of Pt, Ru, or the like. The resistance state is changedby applying a predetermined voltage-current stimulus between theelectrodes 52 and 53 from the outside. Further, even when a power supplyis disconnected, the resistance state is maintained without beingvolatilized.

It is disclosed to use NiO (nickel oxide) as the metal oxide film 51(refer to non-patent document 1). It is proposed to use Ti oxide, Hfoxide, Zr oxide, Zn oxide, W oxide, Co oxide, and Nb oxide as the metaloxide film 51 (refer to patent document 1).

FIG. 11 is a figure showing an example of a current-voltagecharacteristic of an MIM type variable resistance element. FIG. 11( a)shows an electrical characteristic of the variable resistance element ina case in which a voltage that is equal to or higher than Vt1 is appliedto the variable resistance element in the OFF state that is thehigh-resistance state and the state of the variable resistance elementis changed to the ON state that is the low-resistance state. FIG. 11( b)shows an electrical characteristic of a variable resistance element in acase in which a voltage that is equal to or higher than Vt2 is appliedto the variable resistance element in the ON state and the state of thevariable resistance element is changed to the OFF state that is thehigh-resistance state.

In such variable resistance element, a current path in thelow-resistance state is not distributed and formed over the entiresurface of the electrode and is formed locally. A diameter of thecurrent path is about several nm and even in a large case, it is several10 nm.

-   [patent document 1] Japanese Patent Application Laid-Open No.    2008-166768-   [non-patent document 1] Solid State Electronics vol. 7, pp. 785 to    797, (Solid State Electronics, Vol. 7, P. 785-797, 1964.)

BRIEF SUMMARY OF THE INVENTION Problems to be Solved by the Invention

However, in the structure described in patent document 1 and non-patentdocument 1, a problem in which a resistance value of the current paththat is a resistance value in the ON/OFF state greatly varies occurs.

Accordingly, a main purpose of the present invention is to provide avariable resistance element of which variation in the resistance valuein the ON/OFF state is suppressed and a semiconductor storage device.

Means for Solving the Problems

In order to solve the above-mentioned problem, the variable resistanceelement according to the present invention is characterized in that thevariable resistance element is formed by sandwiching a metal oxide layerwhose resistance changes between a pair of electrodes and the metaloxide layer includes a pair of variable resistance layers whoseresistances change by formation of a current path and a branchingsuppression layer which is sandwiched between the variable resistancelayers and suppresses branching of the current path.

Effect of the Invention

In the present invention, because the branching suppression layer isprovided, even when the state of the variable resistance elementtransits to the ON state or the OFF state, the variation in theresistance value at the time of transition can be suppressed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a semiconductor storage device using avariable resistance element according to an exemplary embodiment of thepresent invention.

FIG. 2 is a sectional view of a variable resistance element according toan exemplary embodiment.

FIG. 3 is a flowchart showing a manufacturing procedure of a variableresistance element according to an exemplary embodiment.

FIG. 4 is X-ray diffraction data of a metal oxide film according to anexemplary embodiment.

FIG. 5 is a figure showing a voltage-current characteristic of avariable resistance element produced by using a pure NiO film as a metaloxide film related to a metal oxide film according to an exemplaryembodiment.

FIG. 6 is a figure showing a voltage-current characteristic of avariable resistance element produced by using a metal oxide filmaccording to an exemplary embodiment.

FIG. 7 is a figure showing an initial resistance of a variableresistance element produced by a metal oxide film of which Al or Co isadded to NiO according to an exemplary embodiment.

FIG. 8 is a figure showing a forming voltage of a variable resistanceelement produced by a metal oxide film of which Al or Co is added to NiOaccording to an exemplary embodiment.

FIG. 9 is a figure illustrating an example of a utilization form of avariable resistance element applied to a description of the relatedtechnology.

FIG. 10 is a figure illustrating a structure of a variable resistanceelement applied to a description of the related technology.

FIG. 11 is a figure showing an example of a current-voltagecharacteristic of a variable resistance element applied to a descriptionof the related technology. FIG. 11( a) is a characteristic figureshowing a transition from an OFF state that is a high-resistance stateand FIG. 11( b) is a characteristic figure showing a transition from anON state.

MOST PREFERRED MODE FOR CARRYING OUT THE INVENTION

An exemplary embodiment of the present invention will be described. Thevariable resistance element may be used as not only a switch thatcontrols interconnection between wirings but also a storage element. Inthe following explanation of the exemplary embodiment, a case in whichthe variable resistance element functions as a switch is taken as anexample.

FIG. 1 is a sectional view of a semiconductor storage device using avariable resistance element according to an exemplary embodiment of thepresent invention. This semiconductor storage device 2 includes atransistor 3 and a variable resistance element 4 as a main structure.

The transistor 3 is formed on a semiconductor substrate made of Si orthe like and an interlayer insulating film 31 (31 a, 31 b, and 31 c) isformed on it. A source 32 and a drain 33 of the transistor 3 areconnected to wirings 34 a and 34 b including a metal such as W, Al, Cu,or the like provided through the interlayer insulating films 31 a and 31b.

The variable resistance element 4 connected to the transistor 3 via thewiring 34 b is formed on the interlayer insulating film 31 b.

The variable resistance element 4 is covered by a barrier propertyinsulator 41 and connected to the wiring 34 b and a wiring 42.

FIG. 2 is a detailed sectional view of the variable resistance element4. The variable resistance element 4 is formed by stacking a lowerelectrode portion 15, a metal oxide layer 17, and an upper electrodeportion 19 in this order.

The lower electrode portion 15 includes a barrier layer 15 b and a lowerelectrode 15 a and the upper electrode portion 19 includes an upperelectrode 19 a and a barrier layer 19 b. The metal oxide layer 17includes a variable resistance layer 17 a, a branching suppression layer17 b, and a variable resistance layer 17 c. The composition of thevariable resistance layer 17 a is the same as that of the variableresistance layer 17 c.

Next, a detailed structure of the above-mentioned variable resistanceelement 4 will be described with a manufacturing method. FIG. 3 is aflowchart showing a manufacturing procedure of the variable resistanceelement 4.

Step S1: First, the barrier layer 15 b and the lower electrode 15 a areformed on the wiring 34 from the interlayer insulating film 31 b. Thebarrier layer 15 b includes at least one from among TiN, TaN, WN, MoN,and the like. Further, the lower electrode 15 a includes at least onemetallic element from among the elements such as Ni, Cu, Co, and thelike (hereinafter, described as a first metallic element).

Step S2: Next, the variable resistance layer 17 a is formed. Thevariable resistance layer 17 a includes an oxide of the first metallicelement.

Step S3: The branching suppression layer 17 b is formed on the variableresistance layer 17 a. In the branching suppression layer 17 b, one ofthe first metallic elements such as Ni, Cu, Co, and the like included inthe variable resistance layer 17 a is substituted by one of the elementssuch as Al, Ti, Zr, Hf, Ta, W, Mo, and the like (hereinafter, describedas a second metallic element) that can be ionized into the trivalent ormore state.

Step S4: The variable resistance layer 17 c is formed on the branchingsuppression layer 17 b.

The variable resistance layer 17 a, the branching suppression layer 17b, and the variable resistance layer 17 c can be formed by using a CVD(Chemical Vapor Deposition) method, a sputtering method, or a sol-gelmethod. For example, when the branching suppression layer 17 b is formedby the CVD method, the raw materials including the first metallicelement and the second metallic element are supplied to a reaction tanksimultaneously to form the branching suppression layer 17 b. When thesputtering method is used, the first metallic element and the secondmetallic element are mixed so as to obtain a desired composition ratioand a sintered target is used as the oxide.

A desirable amount of the second metallic element included in thebranching suppression layer 17 b is for example, 0.01 mol % to 50 mol %because its amount does not change a resistance value in thelow-resistance state.

A desirable film thickness of the variable resistance layer 17 cincluding only the first metallic element is 0.1 nm to 3 nm. Further, adesirable film thickness of the variable resistance layer 17 a is 0.1 nmto 1 nm.

Step S5: After the metal oxide layer 17 is formed by the above-mentionprocess, the upper electrode portion 19 is formed by using a method thatis the same as the method used for the lower electrode portion 15.

Step S6: A patterning is performed to a pattern of the variableresistance element 4 by using a photolithography, a dry etching, or thelike.

Step S7: The barrier property insulator 41 is formed. The variableresistance element 4 is covered by this barrier property insulator 41.The barrier layers 15 b and 19 b and the barrier property insulator 41perform an action to suppress the spreading of the element included inthe variable resistance element 4. Si₃N₄, SiCN, SiC, Al₂O₃, or the likeis used.

Thus, in the variable resistance element 4 according to the presentinvention, the branching suppression layer 17 b is sandwiched betweenthe variable resistance layers 17 a and 17 c.

When a voltage is applied to the variable resistance element 4 in aninitial state, a germ of the current path (a starting point of thecurrent path) is formed and the current path is formed when the germgrows. Hereinafter, a voltage required to form a current route is calleda forming voltage. An operation in which the state is first transferredfrom an initial high-resistance state to the low-resistance state isdescribed as a forming process.

Namely, when the forming voltage is applied on the variable resistanceelement, the germ of the current path is generated. A hole and anelectron are injected in this germ or the growing current path. By theinjected carrier, an electric field at a head of the path becomes large(electric field concentration occurs). By this electric fieldconcentration, the current path continuously grows and reaches theelectrode. However, because the state of electric field concentration isa higher energy state, energy dispersion occurs. Namely, the branchingof the current path occurs. When the branching of the current pathoccurs, the low-resistance state occurs according to the branchingstate. Accordingly, in this exemplary embodiment, the branchingsuppression layer 17 b for suppressing the branching of the current pathis provided.

As a parent metal of the branching suppression layer 17 b, a materialthat is the same as that of the variable resistance layers 17 a and 17 cincluding the first metallic element such as Ni, Cu, Co, or the like isused and the branching suppression layer 17 b is formed by using thesecond metallic element such as Al, Ti, Zr, Hf, Ta, W, Mo, and the likethat can be ionized into the trivalent or more state as an additionagent. The first metallic element is substituted by this second metallicelement.

When the first metallic element is substituted by the second metallicelement, a deficit occurs at a substitution site. By this, a quantity ofgenerated holes decreases. Therefore, it is believed that the branchingof the current path is suppressed.

FIG. 4 is X-ray diffraction data indicating that the first metallicelement is substituted by the second metallic element. Further, adesirable addition amount of the second metallic element is several mol% (for example, 5 mol %).

It is required that this branching suppression layer 17 b does notcontact with the upper electrode 19 a. This is because the formingprocess cannot be easily performed when it contacts with the upperelectrode 19. For this reason, it is desirable that the branchingsuppression layer 17 b is arranged at a distance of about 0.1 nm to 3 nmapart from the upper electrode 19 a. In the exemplary embodiment, asmentioned above, the film thickness of the variable resistance layer 17c is set to 0.1 nm to 3 nm in order to separate them from each other.

Further, it is required that the branching suppression layer 17 b doesnot contact with the lower electrode 15 a. In a case in which theforming process is performed by applying a positive voltage on the lowerelectrode 15 a, many germs of the current path required to perform theforming process are formed when an added substance is not used. The highvoltage is required to perform the forming process when the branchingsuppression layer 17 b contacts with the lower electrode 15 a.Therefore, it is important that the branching suppression layer 17 bdoes not contact with the lower electrode 15 a.

Thus branching suppression effect of the current path can be interpretedas follows from a physical property point of view. In order to changethe state of the connection between two wirings L1 and L2 as shown inFIG. 1, a variable resistance element 50 as shown in FIG. 2 is provided.

In order to use this variable resistance element as a switch, thevariable resistance element has to have a large on/off resistance ratioand a resistance value state has to be changed by an appropriateoperation voltage. In particular, a voltage (hereinafter, described as aforming voltage VF) required for first changing the state of thevariable resistance element from the initial high-resistance state tothe low-resistance state is the highest one in the operation. Therefore,the small forming voltage VF is required.

FIG. 5 shows a typical voltage-current characteristic of a publiclyknown MIM type variable resistance element produced by using a pure NiOfilm as a metal oxide film. In contrast, FIG. 6 shows a voltage-currentcharacteristic of an MIM type resistance element according to thepresent invention. Hereinafter, the variable resistance element havingthe characteristic shown in FIG. 5 is described as a publicly knownvariable resistance element and the variable resistance element havingthe characteristic shown in FIG. 6 is described as the variableresistance element of the present invention.

From the characteristic graphs shown in FIG. 5 and FIG. 6, the initialresistance of the variable resistance element of the present inventionis higher than the initial resistance of the publicly known variableresistance element and the forming voltage VF of the variable resistanceelement of the present invention is approximately equal to that of thepublicly known variable resistance element. It is believed that thisdifference arises from the presence of the branching suppression layermentioned above.

FIG. 7 and FIG. 8 are figures showing an initial resistance Rin versusan addition amount and the forming voltage VF versus the addition amountwith respect to the variable resistance element of the present inventionproduced by the metal oxide film of which Co (cobalt) is added to NiOand the publicly known variable resistance element produced by the metaloxide film of which Al (aluminum) is added to NiO, respectively.Further, in FIG. 7 and FIG. 8, the horizontal axis is a ratio of Ni toan addition material (Al or Co) in mol %. The film thickness (it roughlycorresponds to the film thickness of the NiO layer) of each metal oxidefilm is 20 nm.

When Al is added, as shown in FIG. 7, the initial resistance Rin can beincreased by adding several mol percent of the addition material.However, as shown in FIG. 8, the forming voltage VF becomes extremelyhigh.

In contrast, when Co is added, the initial resistance Rin can beincreased like a case of adding Al. However, the increased amount of theinitial resistance Rin when Al is added is a little bit greater than theincreased amount of the initial resistance Rin when Co is added.However, the forming voltage VF at a room temperature when Co is addedis almost equal to the forming voltage VF at a room temperature when AIis added.

The parent metal of the branching suppression layer 17 b including thefirst metallic element (that is Ni in this description) has a NiOcrystal structure. By substituting this Ni element by the secondmetallic element, a large ionic bonding property to an oxygen ion isobtained around the substitution site and the crystal structure becomesstable. Accordingly, the movement of the Ni ion (Ni deficit) requiredfor the growth of the current path scarcely occurs and the branching ofthe current path is suppressed. Even when the deficit of the firstmetallic element such as the Ni element or the like occurs by the secondmetallic element that is the added element, a charge neutral conditioncan be easily satisfied inside the branching suppression layer 17 b.Therefore, the generation of the hole is suppressed. This means that thequantity of the holes that promotes the branching of the current path issuppressed. As a result, the branching of the current path issuppressed. When the branching of the current path is suppressed, thegrowth of the current path disproportionately occurs in the mostelectric field concentrated direction. As a result, the current pathreaches the upper electrode without being branched.

As described above, because the branching of the current path issuppressed by the branching suppression layer, the variable resistanceelement whose characteristic value in the high-resistance state and thelow-resistance state is stable can be obtained.

Further, the invention of the present application is not limited to theabove-mentioned exemplary embodiment (and the example). Various changesin the configuration or details of the invention of the presentapplication that can be understood by those skilled in the art can bemade without departing from the scope of the invention.

This application claims priority based on Japanese Patent ApplicationNo. 2011-060327 filed on Mar. 18, 2011, the disclosure of which ishereby incorporated by reference in its entirety.

DESCRIPTION OF SYMBOL

-   -   2 semiconductor storage device    -   3 transistor    -   4 variable resistance element    -   15 lower electrode portion    -   15 a lower electrode    -   15 b barrier layer    -   17 metal oxide layer    -   17 a variable resistance layer    -   17 b branching suppression layer    -   17 c variable resistance layer    -   19 upper electrode portion    -   19 a upper electrode    -   19 b barrier layer    -   41 barrier property insulator

The invention claimed is:
 1. A variable resistance element, comprising:a metal oxide layer having a variable resistance, sandwiched between apair of electrodes, the metal oxide layer comprising: a pair of variableresistance layers having resistances which change by formation of acurrent path; and a branching suppression layer which is sandwichedbetween the variable resistance layers and suppresses branching of thecurrent path, wherein the variable resistance layer and the branchingsuppression layer include an oxide of a first metallic element thatcomprises at least one metallic element among Ni, Cu, and Co, whereinthe branching suppression layer further comprises a second metallicelement that can be ionized into a trivalent or more state, the firstmetallic element being partially substituted with the second metallicelement, and wherein the second metallic element comprises at least onemetallic element among Al, Ti, Zr, Hf, Ta, W, and Mo.
 2. The variableresistance element described in claim 1, wherein an addition amount ofthe second metallic element is 0.01 mol % to 50 mol %.
 3. The variableresistance element described in claim 1, wherein a film thickness of thevariable resistance layer is set to a range from 0.1 nm to 3 nm.
 4. Thevariable resistance element described in claim 1, wherein a filmthickness of the variable resistance layer is set to a range from 0.1 nmto 1 nm.
 5. The variable resistance element described in claim 1,wherein the variable resistance element is formed so as to sandwich alaminated body formed by stacking the electrode, the metal oxide layer,and the electrode and a barrier layer which restricts diffusion of theelement included in these electrodes and the metal oxide layer isprovided.
 6. A semiconductor storage device, comprising: the variableresistance element described in claim 1; and a transistor connected tothe variable resistance element, wherein an electric current that flowsinto the transistor is limited by the variable resistance element or anoperation state of the transistor is stored.
 7. The semiconductorstorage device described in claim 6, wherein a barrier propertyinsulator which covers the variable resistance element is provided inthe semiconductor storage device.
 8. The variable resistance element ofclaim 1, wherein the pair of electrodes includes the first metallicelement.
 9. A variable resistance element, comprising: a lower electrodeportion; a metal oxide layer having a variable resistance, formed on thelower electrode portion, the metal oxide layer comprising: a firstvariable resistance layer comprising an oxide of a first metallicelement including at least one of Ni, Cu, and Co; a branchingsuppression layer formed on the first variable resistance layer forsuppressing branching of a current path, the branching suppression layercomprising: an oxide of the first metallic element; and a secondmetallic element including at least one of Al, Ti, Zr, Hf, Ta, W, andMo, the first metallic element being partially substituted with thesecond metallic element; and a second variable resistance layer formedon the branching suppression layer and comprising an oxide of the firstmetallic element; and an upper electrode portion formed on the metaloxide layer.
 10. The variable resistance element of claim 9, wherein thelower electrode portion comprises a first barrier layer and a lowerelectrode formed on the barrier layer, and wherein the upper electrodeportion comprises an upper electrode and a second barrier layer formedon the upper electrode.
 11. A semiconductor storage device comprising: atransistor formed on a substrate and including a diffusion region formedin the substrate; an interlayer insulating film formed on the transistorand the substrate; and a variable resistance element formed in theinterlayer insulating film and electrically connected to the diffusionregion of the transistor, the variable resistance element comprising: alower electrode portion; a metal oxide layer having a variableresistance, formed on the lower electrode portion, the metal oxide layercomprising: a first variable resistance layer comprising an oxide of afirst metallic element including at least one of Ni, Cu, and Co; abranching suppression layer formed on the first variable resistancelayer for suppressing branching of a current path, the branchingsuppression layer comprising: an oxide of the first metallic element;and a second metallic element including at least one of Al, Ti, Zr, Hf,Ta, W, and Mo, the first metallic element being partially substitutedwith the second metallic element; and a second variable resistance layerformed on the branching suppression layer and comprising an oxide of thefirst metallic element; and an upper electrode portion formed on themetal oxide layer.
 12. The semiconductor storage device of claim 11,further comprising: a barrier property insulator formed on the variableresistance element.